It is common practice in spectrophotometry to chop the light beam passing through the monochromator with a mechanical chopper having a plurality of apertures to pass the beam alternating with opaque segments to interrupt the beam. This has several advantages. In current digital systems specific photometric tasks can be assigned to handle the light segments of the beam while baseline corrections can be made on the dark intervening segments. These tasks can be assigned by a microcomputer in sequence during the open or light part of the chopper cycle, but must be properly phased or timed to be done on the same part of each light pulse or variations will result from differences in response to pulse shape in successive pulses. This phasing must be locked to the chopper frequency, to a particular time in the chopper cycle, even though the chopper is operated from a synchronous motor, which may be powered by more than one line frequency such as 60 Hz, domestic, or 50 Hz, foreign.
It is an objective of this invention to derive the chopper frequency from the line frequency, as this minimizes stray pickup in the measuring circuits.
It is a further objective of this invention to provide a microcomputer controlled photometric system that derives its timing internally rather than from the line derived chopper frequency for greater timing accuracy.
It is a further objective to initialize or phase this internal timing system at intervals corresponding to the chopper frequency.
It is a related object to phase this internal system to synchronize with a specific occurrence in the chopper light-dark sequence.
It is yet a further object to eliminate the necessity of operator preselection of the mode of microcomputer operation to correspond with a given power line frequency.